| 1. |
- Currie, Thayne, et al.
(författare)
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No Clear, Direct Evidence for Multiple Protoplanets Orbiting LkCa 15 : LkCa 15 bcd are Likely Inner Disk Signals
- 2019
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 877:1
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Tidskriftsartikel (refereegranskat)abstract
- Two studies utilizing sparse aperture-masking (SAM) interferometry and H-alpha differential imaging have reported multiple Jovian companions around the young solar-mass star, LkCa 15 (LkCa 15 bcd): the first claimed direct detection of infant, newly formed planets (protoplanets). We present new near-infrared direct imaging/spectroscopy from the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system coupled with Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS) integral field spectrograph and multi-epoch thermal infrared imaging from Keck/NIRC2 of LkCa 15 at high Strehl ratios. These data provide the first direct imaging look at the same wavelengths and in the same locations where previous studies identified the LkCa 15 protoplanets, and thus offer the first decisive test of their existence. The data do not reveal these planets. Instead, we resolve extended emission tracing a dust disk with a brightness and location comparable to that claimed for LkCa 15 bcd. Forward-models attributing this signal to orbiting planets are inconsistent with the combined SCExAO/CHARIS and Keck/NIRC2 data. An inner disk provides a more compelling explanation for the SAM detections and perhaps also the claimed H-alpha detection of LkCa 15 b. We conclude that there is currently no clear, direct evidence for multiple protoplanets orbiting LkCa 15, although the system likely contains at least one unseen Jovian companion. To identify Jovian companions around LkCa 15 from future observations, the inner disk should be detected and its effect modeled, removed, and shown to be distinguishable from planets. Protoplanet candidates identified from similar systems should likewise be clearly distinguished from disk emission through modeling.
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| 2. |
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| 3. |
- de Val-Borro, M., et al.
(författare)
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A comparative study of disc-planet interaction
- 2006
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 370:529, s. 29-
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Tidskriftsartikel (refereegranskat)abstract
- We perform numerical simulations of a disc-planet system using various grid-based and smoothed particle hydrodynamics (SPH) codes. The tests are run for a simple setup where Jupiter and Neptune mass planets on a circular orbit open a gap in a protoplanetary disc during a few hundred orbital periods. We compare the surface density contours, potential vorticity and smoothed radial profiles at several times. The disc mass and gravitational torque time evolution are analysed with high temporal resolution. There is overall consistency between the codes. The density profiles agree within about 5 per cent for the Eulerian simulations. The SPH results predict the correct shape of the gap although have less resolution in the low-density regions and weaker planetary wakes. The disc masses after 200 orbital periods agree within 10 per cent. The spread is larger in the tidal torques acting on the planet which agree within a factor of 2 at the end of the simulation. In the Neptune case, the dispersion in the torques is greater than for Jupiter, possibly owing to the contribution from the not completely cleared region close to the planet.
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| 4. |
- Izquierdo, Paula, et al.
(författare)
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Fast spectrophotometry of WD 1145+017
- 2018
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 481:1, s. 703-714
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Tidskriftsartikel (refereegranskat)abstract
- WD 1145+017 is currently the only white dwarf known to exhibit periodic transits of planetary debris as well as absorption lines from circumstellar gas. We present the first simultaneous fast optical spectrophotometry and broad-band photometry of the system, obtained with the Gran Telescopio Canarias (GTC) and the Liverpool Telescope, respectively. The observations spanned 5.5 h, somewhat longer than the 4.5-h orbital period of the debris. Dividing the GTC spectrophotometry into five wavelength bands reveals no significant colour differences, confirming grey transits in the optical. We argue that absorption by an optically thick structure is a plausible alternative explanation for the achromatic nature of the transits that can allow the presence of small-sized (~µm) particles. The longest (87 min) and deepest (50 per cent attenuation) transit recorded in our data exhibits a complex structure around minimum light that can be well modelled by multiple overlapping dust clouds. The strongest circumstellar absorption line, Fe II λ5169, significantly weakens during this transit, with its equivalent width reducing from a mean out-of-transit value of 2 to 1 Å in-transit, supporting spatial correlation between the circumstellar gas and dust. Finally, we made use of the Gaia Data Release 2 and archival photometry to determine the white dwarf parameters. Adopting a helium-dominated atmosphere containing traces of hydrogen and metals, and a reddening E(B - V) = 0.01 we find T_eff=15 020 ± 520 K, log g = 8.07 ± 0.07, corresponding to M_WD=0.63± 0.05 M☉ and a cooling age of 224 ± 30 Myr.
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| 5. |
- Liljeström, A. J., et al.
(författare)
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Turbulent stresses as a function of shear rate in a local disk model
- 2009
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Ingår i: Astronomical Notes - Astronomische Nachrichten. - : Wiley. - 0004-6337 .- 1521-3994. ; 330:1, s. 92-99
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Tidskriftsartikel (refereegranskat)abstract
- WC Present local numerical models of accretion disk turbulence driven by the magnetorotational instability with varying shear rate. The resulting turbulent stresses are compared with predictions of a closure model in which triple correlations are modelled in terms of quadratic correlations. This local model uses live nondimensional parameters to describe the properties of the flow. We attempt to determine these Closure parameters for our simulations and find that the model does produce qualitatively correct behaviour. In addition, we present results concerning the shear rate dependency of the magnetic to kinetic energy ratio. We find both the turbulent stress ratio and the total stress to be strongly dependent on the shear rate.
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| 6. |
- Lyra, Wladimir, et al.
(författare)
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An Analytical Theory for the Growth from Planetesimals to Planets by Polydisperse Pebble Accretion
- 2023
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 946:2
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Tidskriftsartikel (refereegranskat)abstract
- Pebble accretion is recognized as a significant accelerator of planet formation. Yet only formulae for single-sized (monodisperse) distribution have been derived in the literature. These can lead to significant underestimates for Bondi accretion, for which the best accreted pebble size may not be the one that dominates the mass distribution. We derive in this paper the polydisperse theory of pebble accretion. We consider a power-law distribution in pebble radius, and we find the resulting surface and volume number density distribution functions. We derive also the exact monodisperse analytical pebble accretion rate for which 3D accretion and 2D accretion are limits. In addition, we find analytical solutions to the polydisperse 2D Hill and 3D Bondi limits. We integrate the polydisperse pebble accretion numerically for the MRN distribution, finding a slight decrease (by an exact factor 3/7) in the Hill regime compared to the monodisperse case. In contrast, in the Bondi regime, we find accretion rates 1-2 orders of magnitude higher compared to monodisperse, also extending the onset of pebble accretion to 1-2 orders of magnitude lower in mass. We find megayear timescales, within the disk lifetime, for Bondi accretion on top of planetary seeds of masses 10−6 to 10−4 M ⊕, over a significant range of the parameter space. This mass range overlaps with the high-mass end of the planetesimal initial mass function, and thus pebble accretion is possible directly following formation by streaming instability. This alleviates the need for mutual planetesimal collisions as a major contribution to planetary growth.
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| 7. |
- Lyra, Wladimir, et al.
(författare)
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Embryos grown in the dead zone : Assembling the first protoplanetary cores in low mass self-gravitating circumstellar disks of gas and solids
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 491:3, s. L41-L44
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Tidskriftsartikel (refereegranskat)abstract
- Context: In the borders of the dead zones of protoplanetary disks, the inflow of gas produces a local density maximum that triggers the Rossby wave instability. The vortices that form are efficient in trapping solids. Aims: We aim to assess the possibility of gravitational collapse of the solids within the Rossby vortices. Methods: We perform global simulations of the dynamics of gas and solids in a low mass non-magnetized self-gravitating thin protoplanetary disk with the Pencil Code. We use multiple particle species of radius 1, 10, 30, and 100 cm. The dead zone is modelled as a region of low viscosity. Results: The Rossby vortices excited in the edges of the dead zone are efficient particle traps. Within 5 orbits after their appearance, the solids achieve critical density and undergo gravitational collapse into Mars sized objects. The velocity dispersions are of the order of 10 m s-1 for newly formed embryos, later lowering to less than 1 m s-1 by drag force cooling. After 200 orbits, over 300 gravitationally bound embryos were formed, 20 of them being more massive than Mars. Their mass spectrum follows a power law of index -2.3 ± 0.2.
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| 8. |
- Lyra, Wladimir, et al.
(författare)
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Fine Structure of the Chromospheric Activity in Solar Type Stars : The H-alpha Line
- 2005
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Ingår i: Astronomy and Astrophysics. ; :431, s. 329-
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Tidskriftsartikel (refereegranskat)abstract
- A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complementing the works previously done on this subject. The chromospheric diagnostic was built with a statistically significant sample, covering nine years of observations, and including 175 solar neighborhood stars. Regarding the age indicator, the presence of stars for which very accurate ages are determined, such as those belonging to clusters and kinematic groups, lends confidence to our analysis. We also investigate the possibility that stars of the same age might have gone through different tracks of chromospheric decay, identifying - within the same age range - effects of metallicity and mass. These parameters, however, as well as age, seem to be significant only for dwarf stars, losing their meaning when we analyze stars in the subgiant branch. This result suggests that, in these evolved stars, the emission mechanism cannot be magnetohydrodynamical in nature, in agreement with recent models. The Sun is found to be a typical star in its Hα chromospheric flux, for its age, mass and metallicity. As a byproduct of this work, we developed an automatic method to determine temperatures from the wings of Hα, which means the suppression of the error inherent to the visual procedure used in the literature.Based on observations collected at Observatório do Pico dos Dias, operated by the Laboratório Nacional de Astrofísica, CNPq, Brazil.
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| 9. |
- Lyra, Wladimir, et al.
(författare)
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Global magnetohydrodynamical models of turbulence in protoplanetary disks : I. A cylindrical potential on a Cartesian grid and transport of solids
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 479, s. 883-901
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Tidskriftsartikel (refereegranskat)abstract
- Aims.We present global 3D MHD simulations of disks of gas and solids, aiming at developing models that can be used to study various scenarios of planet formation and planet-disk interaction in turbulent accretion disks. A second goal is to demonstrate that Cartesian codes are comparable to cylindrical and spherical ones in handling the magnetohydrodynamics of the disk simulations while offering advantages, such as the absence of a grid singularity, for certain applications, e.g., circumbinary disks and disk-jet simulations. Methods: We employ the Pencil Code, a 3D high-order finite-difference MHD code using Cartesian coordinates. We solve the equations of ideal MHD with a local isothermal equation of state. Planets and stars are treated as particles evolved with an N-body scheme. Solid boulders are treated as individual superparticles that couple to the gas through a drag force that is linear in the local relative velocity between gas and particle. Results: We find that Cartesian grids are well-suited for accretion disk problems. The disk-in-a-box models based on Cartesian grids presented here develop and sustain MHD turbulence, in good agreement with published results achieved with cylindrical codes. Models without an inner boundary do not show the spurious build-up of magnetic pressure and Reynolds stress seen in the models with boundaries, but the global stresses and alpha viscosities are similar in the two cases. We investigate the dependence of the magnetorotational instability on disk scale height, finding evidence that the turbulence generated by the magnetorotational instability grows with thermal pressure. The turbulent stresses depend on the thermal pressure obeying a power law of 0.24 ± 0.03, compatible with the value of 0.25 found in shearing box calculations. The ratio of Maxwell to Reynolds stresses decreases with increasing temperature, dropping from 5 to 1 when the sound speed was raised by a factor 4, maintaing the same field strength. We also study the dynamics of solid boulders in the hydromagnetic turbulence, by making use of 106 Lagrangian particles embedded in the Eulerian grid. The effective diffusion provided by the turbulence prevents settling of the solids in a infinitesimally thin layer, forming instead a layer of solids of finite vertical thickness. The measured scale height of this diffusion-supported layer of solids implies turbulent vertical diffusion coefficients with globally averaged Schmidt numbers of 1.0 ± 0.2 for a model with α≈10-3 and 0.78 ± 0.06 for a model with α≈10-1. That is, the vertical turbulent diffusion acting on the solids phase is comparable to the turbulent viscosity acting on the gas phase. The average bulk density of solids in the turbulent flow is quite low (ρp = 6.0×10-11 kg m-3), but in the high pressure regions, significant overdensities are observed, where the solid-to-gas ratio reached values as great as 85, corresponding to 4 orders of magnitude higher than the initial interstellar value of 0.01
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| 10. |
- Lyra, Wladimir
(författare)
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Magnetohydrodynamics of Turbulent Accretion Disks and Planet Formation
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Planets have long been believed to form in disks of gas and dust around youngstars, interacting with their surroundings via a setof complex and highly nonlinear processes. In the core accretion scenario forgiant planet formation dust coagulates first into km-sized icyand rocky planetesimals that further collide, formingprogressively larger solid bodies that eventually give rise to cores of severalEarth masses. If a critical mass is attained, these cores become gas giantplanets by undergoing runaway accretion of gas. Otherwise, just a small amount ofnebular gas is retained by the core, which ends up as an ice giant.Although explaining the overall architecture of the solar system, this picture was shaken by the discovery of the extra-solar planets, which present a diversity far greater than the standard scenario above could predict. Planet-disk interaction seems to be one of the obvious candidates to accountfor this diversity, since planets exchange angular momentum with the disk,leading to either inward or outward migration. An understanding of thephysical state of accretion disks is therefore essential to provide a detailed picture ofthe effect of migration on planetary orbits. Such understanding can be achieved through hydrodynamical modelling of the environment where planets are formed.On the same token, the study of accretion disks is in itself an interesting field of research, since ourcurrent understanding of the physics of accretion is only marginal: the accretion rates observed in circumstellar environments are far too great to be explained by molecular viscosity alone. A milestone was achieved 15 years ago with the realization that the magneto-rotational instability (MRI) generates a vigorous turbulence that appears to be present under a wide range of conditions in accretion disks. Much work was done since in an effort to understand the properties of turbulent magnetized accretion disks and its implications to stellar accretion and planet formation.The equations of hydrodynamics are nonlinear and we must therefore resort to numericalsimulations. For all simulations we employ the \pencilcodec, a 3D high-order finite-difference MHD codeusing Cartesian coordinates. A cylindrical formulation of {\sc Pencil} is presented, but was not yet used in a publication. Planets and stars are treated as particlesevolved with an $N$-body scheme, which is also introduced. In paper I we test the reliability of the numerical setup by contributingto a code comparison project that consisted of performing numerical simulations of a disk-planet systemusing various grid-based andsmoothed particle hydrodynamics codes. Surface density contours, potential vorticityand smoothed radial profiles were compared at several times, as well asthe disk mass and gravitational torque time evolution. Our results were consistent between the manycodes and outperformed the only other Cartesian code used in the comparison.In paper II we develop our models into three-dimensional magnetohydrodynamical simulationsof disks of gas and solids, studying the turbulence generated by the magneto-rotational instabilitythat is believed to be the source of the aforementioned anomalous viscosity. We find that Cartesian gridsare well-suited for accretion disk problems. Thedisk-in-a-box models based on Cartesian grids presented in paper II develop andsustain MHD turbulence, in good agreement with published results achieved withcylindrical codes. We investigate the dependence of the magneto-rotationalinstability on disk scale height, finding evidence that the turbulencegenerated by the magneto-rotational instability grows with thermal pressure. Theturbulent stresses depend on the thermal pressure obeying a power law with index $0.24\pm0.03$, compatible with the value of $0.25$ found in shearing boxcalculations. The ratio of stresses decreased with increasing temperature. Wealso study the dynamics of boulders in the hydromagnetic turbulence. Thevertical turbulent diffusion of the embedded boulders is comparable to theturbulent viscosity of the flow. Significant overdensities arise in the solidcomponent as boulders concentrate in high pressure regions, with interesting implications for planet formation.We went further, on paper III, to study whether the predictions based on local shearing box simulationsare also valid for global reference frames. By doing so, we help on the disentangling of physics andnumerics of simulated hydromagnetic turbulence, since it has been shown that the saturated state of the MRI depends not only on physical parameters, but also on numerical choices such as box length and resolution. We find that the saturation predictor derived from shearing box data also applies for the case of global disks in the minimum saturation regime where the fastest growing wavelength of the MRI is not resolved. A simulation where this wavelength is resolved shows that the scaling law is also accurate for global disks in this regime. Analytical considerations of the ratio of stresses, however, do not agree with the values found in the non-linear saturated state of the turbulence in the global simulations reported here. A brief summary of ongoing projects is presented in the last chapter.
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